The invention relates to a process for manufacturing an air bag fabric.
Air bag fabrics are customarily required to have on the side which faces the occupants of the motor vehicle a low air permeability which should not exceed values of 10 l/dm.sup.2.min for a test pressure differential of 500 Pa. If an air bag is manufactured from such a fabric alone, then the air cushion produced when the air bag is inflated through ignition of the pyrotechnical gas generator is very hard. With a very hard air cushion there is a danger that the driver or front-seat passenger, who was initially violently flung forward at the moment of impact, will be abruptly flung backward, which may give rise to injuries, in particular in the head and neck region.
It is therefore necessary to construct the air bag in such a way that soft cushioning of the vehicle occupants is possible on impact without any danger of rebounding. This can be achieved if the gas which flows into the air bag on release of the air bag function is allowed to partially escape.
U.S. Pat. No. 3,481,625 proposes in relation to this matter that the air bag be provided with holes. However, this means that hot particles will pass from the generator-produced gas into the passenger compartment. They represent a considerable danger to the vehicle occupants.
To avoid the emergence of these particles it was proposed in German Patent No. 36 44 554 that the openings provided for the escaping gas be covered with a filter fabric made of aramid fibers. This requires the appreciable manufacturing expense of sewing in the filter fabric. This additional operation and the high cost of aramid fabrics increase the costs of manufacturing an air bag to such an extent as to make economical production impossible.
It was therefore proposed (Krummheuer W. R., Engineering with Fibres for Airbags, Bag and Belt '90, International Akzo Symposium, Cologne, Apr. 25-27, 1990) to make a two-part air bag, consisting of a contact part and a filter part. The contact part is made of a fabric of very low air permeability (&lt;10 l/dm.sup.2.min). The contact part is that part of the air bag which serves to cushion the vehicle occupants in the event of an accident.
The filter part forms those parts of the inflated air bag which do not face the driver or front-seat passenger. It is made of a fabric of distinctly higher air permeability, which thus makes it possible for the generator-produced gas to escape and filters the emerging gas. Moreover, as the hot gas passes at this point through the fabric, there is also a heat exchange effect, so that the gas passes into the passenger compartment in a somewhat cooler state.
European patent application No. 0 363 490 proposes making a one-part air bag by circular weaving. However, circular-woven air bags, in contradistinction to air bags consisting of two or more parts, do not permit adaptation to the specific vehicle type. For instance, given the present-day requirements relating to the construction of front-seat passenger air bags, it is in fact not possible to make it in one piece; instead, it is absolutely unavoidable that two or more pieces have to be sewn together. Moreover, the necessary sewing-in of holding cords is significantly more difficult in the case of a one-piece air bag than in the case of a two- or multipiece air bag.
International patent application WO 90/09295 likewise describes a one-piece air bag made up of woven fabric components having different air permeabilities. Again, using the process described there it is not possible to manufacture front-seat passenger air bags.
Although air bags made of two or more parts are very adaptable to the requirements of the particular vehicle type and even permit the easy construction of front-seat passenger air bags, the problem arises that two fabrics having very different air permeabilities need to be sewn together. Since the high permeability of the fabric used for the filter part of the air bag is achieved with a less tight weave whereas the contact part of the air bag is made of a tight weave, it is thus necessary here to sew together two fabrics which differ in tightness. As the air bag inflates, it is possible for the less tightly woven fabric to burst open at the seams or for the yarns of the less tightly woven fabric to slip, which may result in an uncontrolled escape of gas.
Moreover, this method of working presents planning problems in weaving, since the air permeability of the filter part must be adapted to the type of vehicle and to the generator used. It is thus necessary for the weaving mill to produce or keep available a large number of fabrics of different tightness.